[circuitslave]

I scanned these comments and didn't see any mention of it - I used to work for a company - http://www.teletrax.tv/ - that used Philips made hardware to encode video watermarks that were not easily detectable. We would then use detector hardware in the top (n) markets of the US and "detect" whenever some channel broadcast a watermarked piece of video. This was usually used to enforce things like Sony licensing you to show a movie of theirs (n) times over a given period of time. There's competitors in that space as well like Nielsen, etc. I am sure HBO is using more than just a visible watermark to track their content.